1) Field of the Invention
The present invention relates to an optical fiber, an optical fiber module, and a Raman amplifier for high-speed optical communications.
2) Description of the Related Art
Raman amplifiers are receiving more attention as an amplifier capable of light amplification in a desired wavelength band and realizing a gain over a bandwidth as broad as 100 nanometers. Any silica-based optical fiber can be employed as an optical fiber used as an amplifying medium of the Raman amplifier. However, as a Discrete Raman amplifier, an optical fiber with Raman gain efficiency (GR/Aeff, where GR is Raman gain coefficient and Aeff is effective area) of 6.5 m/W or more is generally used, and a gain of 20 decibels or more is obtained with a pump light of about 500 milliwatts.
Although the conventional optical fiber achieves the gain of 20 decibels or more with the pump light of about 500 milliwatts, it is accompanied by a large nonlinear phase shift simultaneously, and as a result, communication quality is deteriorated due to a waveform distortion of a signal light, which is an adverse factor that hinders high-speed optical communications.
The nonlinear phase shift of a signal light in an optical fiber with a length L is expressed by
            ΔΦ      NL        ⁡          (      L      )        =                    2        ⁢        π        ⁢                                  ⁢                  n          2                                      λ          S                ⁢                  A          eff                      ⁢                  ∫        0        L            ⁢                                    P            S                    ⁡                      (            z            )                          ⁢                  ⅆ          z                    where Ps is a power of the signal light, λs is a wavelength of the signal light, n2 is a nonlinear refractive index of the optical fiber, and (2π/λ)·(n2/Aeff) is a nonlinear parameter γ of the optical fiber. The above equation tells that, as the nonlinear parameter γ of the optical fiber becomes larger, the nonlinear phase shift of a signal light becomes larger and the nonlinear optical phenomenon such as a self phase modulation or a cross phase modulation becomes easier to occur. Since the nonlinear parameter γ is proportional to the Raman gain efficiency, if the Raman gain efficiency of a high nonlinear optical fiber is increased in order to increase the efficiency of the Raman amplifier, the nonlinear parameter γ is also increased, so that a large nonlinear phase shift occurs.
In other words, a nonlinear phase shaft is more likely to occur as an Aeff or a mode field diameter is small, and a germanium concentration in a center core of the optical fiber is high. Since an optical fiber with a large Raman gain generally has a large nonlinear refractive index n2, a large nonlinear phase shift is caused at the same time.